The invention relates to a process for the preparation of highly concentrated free-flowing aqueous solutions of betaines which contain solids contents of more than 40% by weight, preferably more than 50% by weight.
Betaines have become established in recent years in the cosmetics industry as a regular ingredient of formulations, in particular for hair and body cleansing. They have the ability to form a dense and creamy foam which remains stable over a long period, even in the presence of other surfactants, soaps and additives, associated with cleansing properties which are acknowledged to be good without any irritant side effects, even on sensitive skin.
The preparation of betaines is described in detail in the relevant patent and specialist literature (U.S. application No. 3 225 074). In general, this entails compounds which contain tertiary amine nitrogen atoms being reacted with .omega.-halo carboxylic acids or their salts in aqueous or water-containing media.
The particularly used compounds which contain tertiary amine nitrogen atoms are fatty acid amides of the general formula (II) EQU R.sup.3 --C(O)NH--(CH.sub.2).sub.m --NR.sup.4 R.sup.5 (II)
in which R.sup.3 is the hydrocarbon radical of a fatty acid, which radical is alkyl or alkylene, is straight or branched, and can contain optionally one or more, preferably one to five, double bonds and optionally one to five hydroxyl groups, R.sup.4 and R.sup.5 are identical or different and each is a straight or branched alkyl radical with 1-4 carbon atoms, and m can be 1-3.
In this connection, the alkyl radical R.sup.3 can be derived form the natural or synthetic fatty acids with 6-20 carbon atoms, preferably from the natural vegetable or animal fatty acids with 8-18 carbon atoms, as well as their naturally occurring specifically adjusted mixtures with one another or among one another.
Examples of suitable fatty acids are caproic acid, caprylic acid, captic acid, lauric acid, palmitic acid, stearic acid, linoleic acid, linolenic acid and ricinoleic acid.
The naturally occurring fatty acid mixtures with a mixture of chain lengths that can be 8-18 carbon atoms, such as coconut fatty acid or palm kernel fatty acid, which may optionally be hardened by suitable hydrogenation methods, are preferred.
These fatty acids or fatty acid mixtures are converted by conventional condensation reaction techniques at 140.degree.-200.degree. C. with one or more amines of the general formula (III) EQU H.sub.2 N--(CH.sub.2).sub.m --NR.sup.4 R.sup.5 (III)
in which R.sup.4 and R.sup.5 and m have the meaning specified above with respect to the formula (II), to the fatty acid amides with tertiary nitrogen atoms of the general formula (II).
The subsequent quaternization reaction to give betaines of the formula (IV) EQU R.sup.3 --C(O)NH--(CH.sub.2).sub.m --N.sup.+ R.sup.4 R.sup.5 (CH.sub.2).sub.y COO.sup.-- (IV)
in which R.sup.3, R.sup.4, R.sup.5 and m have the same meaning as for the formulae (II) and (III) and y can be 1, 2, 3, can be carried out by processes known from the literature.
As a rule, this entails adding to the fatty acid amide of the formula (II) in aqueous medium one or more .omega.-haloalkylcarboxylic acids, preferably chloroacetic acid, or the salt thereof, preferably the sodium salt, and completing the quaternization in a reaction at about 80.degree.-100.degree. C. for several hours. Depending on the fatty acid or fatty acid mixture used it can be necessary, in order to maintain stirrability as the reaction advances, for a minimum amount of water to be present. The commercially customary concentration of betaine content in the solutions prepared in this way is therefore about 30% by weight or below.
To save storage and transport costs as well as for formulation technological reasons in further processing, however, in many cases a higher concentration has been urgently required.
In the past therefore a number of processes intended to solve this problem have been proposed. Thus, DE-C 3 613 944 discloses a process in which the quaternization is carried out in an organic polar solvent with a water content of 20% by weight, and then the solvent is removed wholly or partly by distillation, and then the desired concentration is adjusted again with an industrially utilizable solvent.
Apart from the fact that the process is industrially elaborate and cost-intensive, organic solvents are in many cases undesired in the further processing to cosmetic formulations.
Although the process disclosed in DE-C 3 726 322 does without organic solvents, the amount of water needed for the quaternization reaction must be removed again from the reaction product by distillation, and the pH of the solution must be adjusted by relatively large amounts of acid to values of 1-4.5, which are untypical of the skin, before or after the adjustment to the desired concentration.
According to EP-A-0 353 580, nonionic, water-soluble surfactants are added to the reaction mixture composed of fatty acid amide and haloalkylcarboxylic acid before or during the quaternization reaction or to the resulting solution of the betaine, in amounts such that the finished solution contains 3-20% by weight of water-soluble surfactants.
This patent discloses that polyoxyethylene ethers are used as the nonionic surfactants and must, for adequate solubility in water, contain 10-250 oxyethylene units.
Polyoxyethylene ethers with relatively high contents of oxyethylene units have, however, proven to be not without problems in respect of their biodegradability.
There has therefore continued to be a need for highly concentrated, free-flowing and pumpable, aqueous solutions of betaines which are free of lower alcohols such as methanol, ethanol, propanol or isopropanol.